The present invention relates to the fabrication of three-dimensional objects using additive process modeling techniques. More particularly, the invention relates to modeling machines which form three-dimensional objects by depositing modeling material onto a substrate mounted to a modeling platform.
Additive process modeling machines make three-dimensional models by building up a modeling medium, usually in planar layers, based upon design data provided from a computer aided design (CAD) system. A mathematical description of a physical part to be created is split into (usually) planar layers, and those layers are individually shaped and applied to produce the final part. Three-dimensional models are used for functions including aesthetic judgments, proofing the mathematical CAD model, forming hard tooling, studying interference and space allocation, and testing functionality. The dominant application of layered manufacturing in recent years has been for rapid prototyping.
Examples of apparatus and methods for making three-dimensional models by depositing solidifiable modeling material are described in Crump U.S. Pat. No. 5,121,329, Batchelder, et al. U.S. Pat. No. 5,303,141, Crump U.S. Pat. No. 5,340,433, Batchelder, et al. U.S. Pat. No. 5,402,351, Crump et al. U.S. Pat. No. 5,503,785, Abrams et al. U.S. Pat. No. 5,587,913, Danforth, et al. U.S. Pat. No. 5,738,817, Batchelder, et al. U.S. Pat. No. 5,764,521 and Comb et al. U.S. Pat. No. 5,939,008, all of which are assigned to Stratasys, Inc., the assignee of the present invention. An extrusion head extrudes solidifiable modeling material in a fluent strand (also termed a xe2x80x9cbeadxe2x80x9d or xe2x80x9croadxe2x80x9d) from a nozzle onto a base. The base comprises a modeling substrate which is removably affixed to a modeling platform. The extruded material is deposited layer-by-layer in areas defined from the CAD model, as the extrusion head and the base are moved relative to each other by mechanical means in three dimensions. The finished model is removed from the substrate. A solidifiable material which adheres to the previous layer with an adequate bond upon solidification is used as the modeling material. Thermoplastic materials have been found particularly suitable for these deposition modeling techniques.
Other additive process manufacturing techniques include depositing UV curable polymers as in Masters U.S. Pat. No. 5,134,569; jetting of droplets of material as in Helinski U.S. Pat. No. 5,136,515; extruding a settable plastic in vertical strips as in Valaaara U.S. Pat. No. 4,749,347; laser welding deposition as in Pratt U.S. Pat. No. 5,038,014; stacking and adhering planar elements as in DiMatteo U.S. Pat. No. 3,932,923; and applying shaped layers of paper as in Hull U.S. Pat. No. 5,192,559.
In additive process three-dimensional modeling machines utilizing manufacturing techniques such as those described above, the model is built up on a base typically comprising a substrate mounted on a modeling platform. The material being deposited must adhere to the substrate to form a foundation layer over which the remaining layers of the object are deposited. The substrate stabilizes the model as it is built up, and facilitates removal of the model from the modeling machine when the model is complete.
It is preferred that parts deposited on the modeling substrate be strongly adhered thereto to overcome two effects. First, strains generated within the extruded material tend to warp the deposited structures unless the structures are supported in their correct orientation. The substrate is important in serving to avoid localized shrinkage in the foundation layer. Second, in some deposition processes, there are forces such as pull from an extrusion nozzle and centripetal acceleration on parts that are not stationary, that tend to distort the deposited structures. A delamination of the foundation layer from the substrate during the building of the object could result in a total failure in forming the object. Further, since the removable substrate becomes a defining surface for the object being built, it must be held in a well-defined configuration. Typically, the substrate is held in a configuration approximating a plane.
The Crump ""329 and ""433 patents disclose a foam plastic material for use as a modeling substrate.: A blue polystyrene material manufactured by Dow-Corning Corp under that name and having a compression strength of 30 psi is identified as particulary suitable coarse, porous structure. The Crump ""329 and ""433 patents also disclose modeling on a wire mesh sandpaper substrate, and on a water soluble wax. The Batchelder et al. ""521 patent discloses a sheet of magnetic material for use as a modeling substrate, wherein the modeling platform includes a magnet for attracting the sheet, while the Comb ""008 patent discloses a flexible sheet substrate held down by vacuum forces.
In rapid prototyping systems sold in the past by Stratasys, Inc., a preferred substrate material has been a polymer foam. A foam slab substrate has proven particularly suitable for supporting models made by extrusion-based deposition modeling techniques. The porosity and compressibility of foam allows foundation layers of modeling material to be buried into the foam, which increases stability of the model as is it built up.
Before building up a model in commercially available modeling machines which build a model on a substrate mounted to a platform which moves along a z-axis, the z-axis position of the platform requires initialization. For example, in the Stratasys FDM(copyright) modeling machines, the operator manually moves the modeling platform up or down to place an extrusion nozzle at a correct position with respect to the substrate. Nozzle tip depth is typically set at one location of the substrate. In the Stratasys FDM(copyright) machines that built the model on a foam slab substrate, the optimal position of the extrusion nozzle is below the top surface of the foam, at a depth so that the first two extruded layers of modeling material will be buried in the foam. The first two layers extruded into the foam substrate provide a foundation to stabilize a model as it is built up. The z-axis accuracy for building on the foam should be xc2x15-10 mils. For rigid substrates, the z-axis accuracy desired in a Stratasys FDM(copyright) modeling system is xc2x12-4 mils.
Unfortunately, achieving the correct position of a modeling platform by a manual or xe2x80x9ceyeballxe2x80x9d method can be unreliable and time-consuming. As proper positioning requires operator judgment, it can be difficult for inexperienced users to identify the correct position. In the Stratasys FDM(copyright) machines, if the top surface of the substrate is not in an xy plane, nozzle tip depth will be incorrect at some locations of the substrate. This results in xe2x80x9cmodeling in airxe2x80x9d or in too much xe2x80x9cplowingxe2x80x9d by the tip.
An incorrect z-start position can result in failure in forming a model. Additionally, if the substrate is not mounted flat to the platform, or if the substrate is deformed or improperly mounted so that it lacks a horizontal planar surface, the model quality will be adversely affected regardless of the z-start position selected. There exists a need for a z-axis initialization routine that does not require operator intervention or judgment.
The present invention is an automated apparatus and method for initialization of a computer-controlled modeling machine that builds up three-dimensional objects on a substrate supported by a modeling platform.
An autoinitialization method of the present invention will calculate a z-start position for the platform based upon z-axis positions of the platform obtained by sensing the substrate surface at multiple x, y positions. A method for determining the z-start position includes the steps of (a) positioning a plunging means above a pre-selected x, y location of a substrate mounted on the platform; (b) raising the platform to drive the substrate against the plunging means; (c) electrically detecting contact by the substrate against the plunging means and responsive providing a detection indication; (d) responsive to the detection indication of step (c), electrically recording the z-axis position of the platform corresponding to the detected contact between the substrate and the plunging means; (e) repeating steps (a) through (d) at a plurality of other pre-selected x, y locations of the substrate to obtain a plurality of recorded z-axis positions; and (f) using the plurality of recorded z-axis positions to calculate a z-start position of the platform. The modeling machine can then automatically position the platform at the calculated z-start position, without operator intervention. In optional additional steps, the method compares the recorded z-axis positions to verify that the substrate top surface is parallel to an xy-plane, and notifies the operator to re-load the substrate if the substrate surface does not approximate a horizontal plane. Or, step (f) can include a step of performing calculations based on the recorded z-axis positions to define a coordinate system fitted to the substrate surface. The model can then be built up in the coordinate system of the substrate.
In an alternative embodiment, the autoinitialization method of the present invention is used to determine whether a previously built model remains in the modeling machine by sensing and evaluating the z-height of the substrate or object carried by the platform. In a further alternate embodiment, the method is used to determine coordinates at which to continue building a model in the event of a power outage during the build process.
A sensor assembly for use in performing the method of the present invention includes a plunger, a sensor and an actuator. The plunger has a raised stored position and a lowered actuated position. The actuator switches the plunger from its stored position to its actuated position for use in executing the z-axis initialization routine and switches the plunger back to its stored position upon completion of the routine. In its actuated position, the plunger is movable by application of an upward force to a sensing location a predetermined distance above the actuated position. The sensor detects the presence of the plunger at the sensing location, providing an output signal when the plunger is detected. Upward force by the substrate against a downward facing tip of the plunger will drive the plunger up the predetermined distance. The sensor assembly can be mounted to the extrusion head of an extrusion-based modeling machine.